Arrange potato evenly on a lightly greased 15- x 10-inch jellyroll pan; coat potato with cooking spray.
Bake at 475°, stirring occasionally, 20 to 25 minutes or until tender and golden.
Whisk together lemon juice and next 5 ingredients; whisk in oil in a slow, steady stream. Gently toss potato and onion with 1/2 cup of vinaigrette.
Arrange spinach evenly in 6 bowls, and drizzle with remaining vinaigrette. Top with potato mixture; sprinkle with bacon.
*2 1/2 pounds small new potatoes may be substituted.